1. Field of the Invention
The present invention relates to a liquid crystal display device, and more in particular to alignment treatment technique of alignment layers of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device comprises a pair of opposed and spaced substrates, an electrode and an alignment layer formed on one of the substrates, an electrode and an alignment layer formed on the other substrate, and a liquid crystal inserted between the pair of the substrates. The electrode on one of the substrates is a common electrode, and the electrode on the other substrate is formed as pixel electrodes. The pixel electrodes can be arranged with an active matrix. Also, the electrode may be arranged only on one of the substrates (in IPS mode, for example). A black matrix and a color filter are arranged on one of the substrates.
In the conventional TN liquid crystal display device, the alignment layer is rubbed so that the liquid crystal aligns in a predetermined direction. The rubbing is a treatment to rub the alignment layer with a cloth as Rayon, and dust occurs as the cloth of Rayon or the like is brought into the clean room. Also, the rubbing generates static electricity which may destroy the TFT (thin film transistors) of the active matrix. In view of this, JP-A-9-354940 which is an application filed prior to the present application proposes to carry out the alignment treatment by the irradiation of ultraviolet light. The alignment treatment with the irradiation of ultraviolet light can obviate the problem of rubbing. According to that proposal, the unpolarized ultraviolet light is irradiated on the alignment layer having a vertical alignment property, and the alignment layer is aligned so that the alkyl side chain realizing the vertical alignment is only partly destroyed. In the case of a horizontal alignment layer, however, the alignment treatment by irradiation of the ultraviolet light is difficult to realize.
In the case where the alignment layer having the vertical alignment property is treated to realize alignment,by irradiating the ultraviolet light, the degree of the alignment depends on the exposure energy of the ultraviolet light to a large measure. An examination shows that it is desirable to irradiate the ultraviolet light of a proper exposure energy. In the case where the exposure energy is smaller than the proper amount, for example, the. pretilt angle established by the alignment layer is high (nearer to the normal to the substrate surface). When distributing spacers for maintaining the cell gap of the liquid crystal panel, therefore, the alignment of the portion around the spacers is disturbed. The result is a display fault with an alignment in which a black spot is generated around the spacers at the time of display.
In the case where the exposure energy is larger than the proper amount, in contrast, the pretilt angle established by the alignment layer is low. The excessive exposure energy, however, may make a horizontally-aligned portion and a portion with a filling streak caused at the time of filling liquid crystal, thus leaving a trace. Another problem is that a desired vertical alignment cannot be obtained but a horizontal alignment develops.
Also, the ultraviolet light cannot be irradiated on the surface of the alignment layer at any angle. In the case where the ultraviolet light is irradiated at an angle almost perpendicular to the surface of the alignment layer, for example, it is impossible to selectively leave only a portion of the alkyl side chain of the alignment layer and hence impossible to obtain an alignment in the desired direction.
The object of the present invention is to provide a method of fabricating a liquid crystal display device by which it is possible to carry out-a proper alignment treatment, without rubbing.
According to the present invention, there is provided a method of fabricating a liquid crystal display device comprising a pair of opposed and spaced substrates, an alignment layer formed on one of the substrates, an alignment layer formed on the other substrate, and a liquid crystal inserted between the pair of substrates, the method comprising the steps of forming an alignment layer including a polymer realizing a vertical alignment on each of the substrates, and irradiating the surface of each of the alignment layers with unpolarized ultraviolet light having an exposure energy of 30 to 120 mJ/cm2 per one pecrent of the polymer content realizing the vertical orientation of the alignment layer in the oblique direction at an angle not more than 45 degrees with respect to the surface of the alignment layers.
The present invention realizes an alignment with a pretilt angle by obliquely irradiating the surface of the vertical alignment layer with the unpolarized ultraviolet light, without rubbing. In the irradiation of the unpolarized ultraviolet light according to the present invention, the unpolarized ultraviolet light is irradiated in the oblique direction at 45 degrees or less with respect to the surface of the alignment layer with an exposure energy of 30 to 120 mJ/cm2 per one percent of polymer content having a side chain realizing the vertical alignment of the alignment layer. In other words, the amount of the irradiation of the ultraviolet light is set in accordance with the actual amount of the side chain realizing the vertical alignment of the alignment layer.
As a result, the alignment layer can provide a proper pretilt angle adapted for vertical alignment of the liquid crystal, without misorder in the alignment around the spacers or without transferring to the horizontal alignment. Under this condition, even if the amount of polymer having side chain contained in the alignment layer for realizing the vertical alignment changes, the proper pretilt angle can be obtained by changing the exposure energy accordingly.
In the above-mentioned method, preferably, the exposure energy of the unpolarized ultraviolet light irradiated on the surface of the alignment layer is in the range of 40 to 90 mJ/cm2.
Also, the exposure energy of the unpolarized ultraviolet light irradiated on the surface of the alignment layer is desirably in the range of 80 to 120 mJ/cm2.
Further, the exposure energy of the ultraviolet light is determined in such a manner that the pretilt angle of the liquid crystal with respect to the surface of the alignment layer is 89.5 degrees or less. Preferably, the pretilt angle of the liquid crystal is in the range of 89.5 to 89 degrees.
Preferably, the ultraviolet light irradiated on the alignment layer contains a component having wavelength of not more than 280 nm.